The present invention relates, in general, to a process for the preparation of silicon wafers having a controlled distribution of oxygen precipitate nucleation centers. More particularly, the present invention is for the preparation of wafers having a non-uniform distribution of oxygen precipitate nucleation centers wherein the distribution is such that, upon a subsequent oxygen precipitation heat treatment, the wafers form a denuded zone in the region near the surface of the wafer and oxygen precipitates in regions outside the denuded zone.
Single crystal silicon, the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared with the so-called Czochralski process wherein a single seed crystal is immersed into molten silicon and then grown by slow extraction. At the temperature of the silicon molten mass, oxygen comes into the crystal lattice from the quartz crucible in which it is held until it reaches a concentration determined by the solubility of oxygen in silicon at the temperature of the molten mass and by the actual segregation coefficient of oxygen in solidified silicon. Such concentrations are greater than the solubility of oxygen in solid silicon at the temperatures typical for the processes for the fabrication of integrated circuits. As the crystal grows from the molten mass and cools, therefore, the solubility of oxygen in it decreases rapidly, whereby in the resulting slices or wafers, oxygen is present in supersaturated concentrations.
Thermal treatment cycles which are typically employed in the fabrication of electronic devices can cause the precipitation of oxygen in silicon wafers which are supersaturated in oxygen. Depending upon their location in the wafer, the precipitates can be harmful or beneficial. Oxygen precipitates located in the bulk of the wafer are capable of trapping undesired metal impurities that may come into contact with the wafer. The use of oxygen precipitates located in the bulk of the wafer to trap metals is commonly referred to as internal or intrinsic gettering ("IG"). Oxygen precipitates located in the active device region of the wafer (within a few microns of the wafer's polished surface), however, can severely degrade device performance. Therefore, it is desirable to create a highly non-uniform distribution of these precipitates. The region near the front (polished) surface of the silicon wafer should contain a minimum density of these oxygen precipitates. The density of precipitates should then increase sharply at some distance (about 20-200 microns) from the surface. The precipitate free region near the surface is generally referred to as the "denuded zone." This zone is a critical parameter in the manufacture of integrated circuits ("IC").
Historically, depth distribution of oxygen precipitates has been controlled by insuring that a sufficient amount of oxygen out-diffuses to the wafer surface and is removed prior to the point at which significant precipitation begins to occur within the wafer. This out-diffusion process requires relatively long heat treatments at very high temperatures, e.g., about 16 hours at 1100.degree. C. This out-diffusion step is then usually followed by a heat treatment (e.g., about 4 hours at 700.degree. C.) to accelerate the subsequent precipitation of oxygen in those regions still containing sufficient amounts of oxygen.
A critical requirement for many electronic device fabricators is that all wafers subjected to this thermal sequence have a uniform and reproducible denuded zone and a uniform and reproducible number density of oxygen precipitates outside the denuded zone. Uniformity and reproducibility have been difficult to achieve at a reasonable cost, however. There are several parameters which affect the density of oxide precipitates which develop in a given silicon wafer in a given IC manufacturing process, including: (1) the concentration of interstitial oxygen, O.sub.i !.sub.i present initially in solid solution, (2) the density of pre-existing (to the IC manufacturing process) oxygen clusters which act as nucleation sites for the precipitation of supersaturated oxygen, (3) the stability of these pre-existing clusters at higher temperatures, and (4) the details of the thermal cycles employed to produce the electronic device. These parameters can vary significantly from one wafer to the next.
One approach which has been tried to control the range of the concentration of oxygen precipitates formed during an IC manufacturing process is to narrow the range of oxygen concentration for the wafers. For example, many IC fabricators require that the range of oxygen concentration be within 1 ppma of a target value, or even less. This approach, however, stretches technological capability, reduces the flexibility of crystal growers to control other parameters and increases costs. Even worse, tightening oxygen concentration specifications does not guarantee success; thermal histories of the silicon wafers can have a profound effect upon the oxygen precipitation behavior. Thus, wafers having the same oxygen concentrations but different thermal histories can exhibit significantly different precipitate densities.
In view of the fact that tightening oxygen concentration specifications by itself will not lead to a narrow range of oxygen precipitate densities, some have attempted sorting wafers by oxygen concentration or other criteria from which values of oxygen precipitation values can be predicted. See, for example, Miller U.S. Pat. No. 4,809,196. Wafer-to-wafer uniformity with respect to oxygen precipitation is improved by this approach, but flexibility is significantly impaired and costs are increased.
Bischoff et al. suggest a process for forming wafers having a wide denuded zone (.gtoreq.15 .mu.m) with a high precipitate density (&gt;10.sup.12 /cm.sup.3) in U.S. Pat. No. 4,437,922. In their process, the denuded zone is formed first by annealing the wafers at 1100.degree. C. for four hours. After the denuded zone is formed, Bischoff et al. suggest that the wafers be annealed at temperature in the range of 400.degree. to 500.degree. C. to nucleate a high density of very small precipitates and grow them to such a size to permit survival of a subsequent heat treatment such as 925.degree. C. Thereafter, Bischoff et al. suggest heating the wafers at a rate of less than 2.degree. C. per minute to a temperature between 750.degree. C. and 1000.degree. C. and annealing the wafers at this temperature for a period which is sufficient to ensure the survival of the precipitates in subsequent processing. These steps add significant labor and cost to the wafers.